A common fault analysis of temperature instrument system
(1) Sudden increase in temperature: This fault is mostly caused by thermal resistance (thermocouple) open circuit, loose wiring terminals, (compensation) wire break, temperature failure, etc. At this time, you need to understand the location of the temperature and the wiring layout. Using the resistance (millivolt) file of a multimeter to measure several sets of data at different positions can quickly find the cause.
(2) Sudden decrease in temperature: This fault is mostly caused by thermocouple or thermal resistance short circuit, wire short circuit and temperature failure. Start with the weak points that are prone to failure, such as wiring ports and wire turns, and investigate them one by one. The temperature rises on site, but the general control indication remains unchanged, mostly due to liquid (water) with a lower boiling point at the measuring element.
(3) Large fluctuations or rapid fluctuations in temperature: At this time, the process operation should be mainly inspected (inspection and adjustment system involved in adjustment).
Two common faults and analysis of pressure instrument system
(1) The pressure suddenly decreases or increases or the indication curve does not change: At this time, check the pressure impulse system of the transmitter, check whether the root valve is blocked, whether the impulse pipe is unblocked, whether there is abnormal medium or sewage wire in the impulse pipe Whether the plugging and drain valve is leaking, etc. Medium freezing in winter is also a common phenomenon. The transmitter itself is unlikely to fail.
(2) Large pressure fluctuations: This situation must first be combined with the craftsman, which is generally caused by improper operation. The parameters involved in the adjustment should mainly check the adjustment system.
Three common faults and analysis of flow meter system
(1) The minimum flow indication value: Generally caused by the following reasons: the detection element is damaged (zero point is too low.; the display is faulty; the line is short-circuited or open; the positive pressure chamber is blocked or leaked; the system pressure is low; the parameters involved in the adjustment should be checked Regulators, regulating valves and solenoid valves.
(2) The maximum flow indication: the main reason is the blocking or leakage of the negative pressure chamber pressure system. It is unlikely that the transmitter needs to be adjusted.
(3) Large flow fluctuations: the flow parameters do not participate in the adjustment, usually because of the process; those involved in the adjustment, can check the PID parameters of the regulator; with the parameters of the isolation tank, check whether there are bubbles in the pressure pipe, positive and negative pressure Is the liquid in the tube the same height?
Four common faults and analysis of liquid level instrument system
(1) The liquid level suddenly increases: Mainly check whether the pressure system of the negative pressure chamber of the transmitter is blocked, leaked, gas collected, or lack of liquid. The specific method of filling liquid is: stop the watch first in the order of stopping the watch; close the positive and negative pressure root valve; open the positive and negative pressure drain valve to relieve pressure; open the double-chamber balance container filling plug; open the positive and negative pressure chamber drain plug; At this time, the liquid level indicator is the maximum. Close the drain valve; close the drain plug in the positive and negative pressure chamber; slowly pour the same medium into the double-chamber balance container, then slightly open the drain plug to exhaust; until it is full, open the plug in the positive pressure chamber and change The transmitter indicates that it should return to zero. Then use the transmitter according to the order of the meter.
(2) The liquid level suddenly decreases: mainly check whether the positive pressure chamber pressure system is blocked, leaked, gas-collected, lack of liquid, and the balance valve is closed. The specific method to check whether the pressure system is unblocked is to stop the transmitter, open the drain valve, and check the drain condition (except for the medium that cannot leak).
(3) The instructions of the master control room do not match the on-site liquid level: first determine whether the on-site liquid level gauge is faulty. At this time, the liquid level can be artificially increased or lowered, and the cause of the problem can be specifically analyzed according to the on-site and master control instructions (on-site liquid level gauge) The root valve is closed, blocked, and leaks easily to cause inaccurate on-site instructions). The normal liquid level can be restored by checking the zero point, range, and filling. If it is still not normal, you can notify the process personnel to monitor on-site to remove the transmitter to suppress the pressure adjustment.
(4) Frequent liquid level fluctuations: First, check the feeding and discharging conditions with the process personnel, and after confirming that the process is normal, it can be stabilized by adjusting the PID parameters. The specific method is: adjust the control valve to manual state, first adjust the setting value to be consistent with the measured value to stabilize the fluctuation of the liquid level, and then slowly adjust the opening of the control valve to make the liquid level rise or fall slowly to meet the process requirements, and then adjust The set value is consistent with the measured value, and the regulating valve will automatically switch after the parameter is stable.
In short, once the instrument parameters are found to be abnormal, first, combine with the process personnel, start with the process operating system and the field instrument system, comprehensively consider, carefully analyze, and especially consider the correlation between the measured parameter and the control valve, and divide the fault into account. Step by step, it is easy to find the problem and solve the problem with the right medicine.
2. On-site control instruments are mainly valves
Valve safety functions and uses can be divided into the following categories
1. Exhaust valve: Exclude excess gas in the pipeline, improve the efficiency of pipeline use and reduce energy consumption.
2. Diversion valve: distribute, separate or mix the medium in the pipeline.
3. Safety valve: prevent the medium pressure in the pipeline or device from exceeding the specified value, so as to achieve the purpose of safety protection.
4. Check valve: prevent the medium in the pipeline from flowing back.
5. Cutoff valve: connect or cut off the flow of medium in the pipeline.
6. Regulating valve: Regulate the pressure, flow and other parameters of the medium.
Now mainly introduce the self-supporting control valve and pneumatic control valve.
One self-operated pressure regulating valve
1. Working principle of self-operated pressure regulating valve (pressure control behind the valve)
The pressure P1 before the valve of the working medium changes to the pressure P2 after the valve after the valve core and valve seat are throttled. P2 is input into the lower diaphragm chamber of the actuator through the control line and acts on the top plate. The force generated is balanced with the reaction force of the spring, which determines the relative position of the valve core and valve seat, and controls the pressure behind the valve. When the pressure P2 behind the valve increases, the force P2 acts on the top plate also increases. At this time, the acting force of the top plate is greater than the reaction force of the spring, so that the valve core is closed to the position of the valve seat until the acting force of the top plate is balanced with the reaction force of the spring. At this time, the flow area between the valve core and the valve seat is reduced, and the flow resistance becomes larger, so that P2 is reduced to the set value. In the same way, when the pressure P2 after the valve decreases, the direction of action is opposite to the above. This is the working principle of the self-operated (after the valve) pressure regulating valve.
2. Working principle of self-operated pressure regulating valve (pressure control in front of the valve)
The pressure P1 before the valve of the working medium changes to the pressure P2 after the valve after the valve core and valve seat are throttled. At the same time, P1 is input into the upper diaphragm chamber of the actuator through the control line and acts on the top plate. The force generated is balanced with the reaction force of the spring, which determines the relative position of the valve core and valve seat, and controls the pressure in front of the valve. When the pressure P1 in front of the valve increases, the force P1 acts on the top plate also increases. At this time, the force of the top plate is greater than the reaction force of the spring, so that the valve core moves in the direction away from the valve seat until the force of the top plate is balanced with the reaction force of the spring. At this time, the flow area between the valve core and the valve seat is reduced, and the flow resistance becomes smaller, so that P1 is reduced to the set value. Similarly, when the pressure P1 in front of the valve decreases, the direction of action is opposite to the above. This is the working principle of the self-operated (in front of the valve) pressure regulating valve.
3. Working principle of self-operated flow control valve
After the controlled medium is input to the valve, the pressure P1 before the valve is input into the lower membrane chamber through the control line, and the pressure Ps after being throttled by the throttle valve is input into the upper membrane chamber. The difference between P1 and Ps, namely â–³Ps=P1-Ps, is called the effective pressure . The difference between the thrust generated by P1 on the diaphragm and the thrust generated by Ps on the diaphragm is balanced with the spring reaction force to determine the relative position of the valve core and the valve seat, thereby determining the flow rate through the valve. When the flow through the valve increases, that is, â–³Ps increases. As a result, P1 and Ps act on the lower and upper membrane chambers respectively, causing the valve core to move toward the valve seat, thereby changing the flow area between the valve core and the valve seat, so that When Ps increases, the thrust of the increased Ps on the diaphragm plus the spring reaction force and the thrust of P1 on the diaphragm create a balance in the new position to achieve the purpose of flow control. On the contrary, the same is true.
Two pneumatic control valve
Pneumatic control valve uses compressed air as power source, cylinder as actuator, and uses electrical valve positioners, converters, solenoid valves, limit valves and other accessories to drive the valve to achieve on-off or proportional adjustment, and receive industrial The control signal of the automation control system completes the adjustment of various process parameters such as flow, pressure, temperature and liquid level of the pipeline medium.
1. Classification of pneumatic control valves
The action of pneumatic control valve is divided into two types: air-open type and air-close type. Air to Open means that when the air pressure on the membrane head increases, the valve moves in the direction of increasing opening. When the upper limit of the input air pressure is reached, the valve is fully open. Conversely, when the air pressure decreases, the valve moves in the closing direction. When there is no air input, the valve is fully closed. Therefore, sometimes the air-to-open valve is also called Fail to Close FC. The air to close type (Air to Close) moves in the opposite direction to the air open type. When the air pressure increases, the valve moves in the closing direction; when the air pressure decreases or does not, the valve moves in the opening direction or fully open. Therefore, it is sometimes called Fail to Open FO. The air opening or closing of the pneumatic control valve is usually realized through the positive and negative effects of the actuator and the different assembly methods of the valve structure.
2. Several common professional terms
The regulating valve is composed of two parts: an actuator and a valve body. Regulating valves generally use pneumatic membrane actuators, and their action modes are positive and negative. When the signal pressure increases, the push-dryer moves down as a positive-acting actuator; when the signal pressure increases, the push-dryer moves up as a counteracting actuator. The valve body parts are divided into two types: positive and negative. When the valve stem moves down, the flow area between the valve core and the valve seat is reduced for the front-mounted type, and vice versa. The function of the regulating valve is divided into two types: air opening and air closing. Air opening and air closing are combined by the positive and negative effects of the actuator and the positive and negative assembly of the valve body parts.
The air-opening or air-closing of the regulating valve is comprehensively considered in many aspects. First, the process safety is the main consideration. After determining the air-closing or air-opening, determine the role of the actuator, and finally determine the front and back of the valve body. The combination method is as described above.
The positive acting actuator means that when the gas pressure on the diaphragm increases, the actuator push rod moves toward the valve body; the counteracting actuator means that when the gas pressure on the diaphragm increases, the actuator push rod moves away from the valve body; and the air opens (air to open) and air to close valves are completely different concepts. The valve of the positive acting actuator and the positive installation (reverse installation) can be air-closed (air opening); on the contrary, the valve of the counteracting actuator and the reverse installation (direct installation) can be air-closed (air opening).
The positive and negative effects of the positioner correspond to the air opening and air closing of the regulating valve you choose. That is to say in order to realize the negative feedback of the entire valve itself. The positive and negative effects of the regulator are set for the negative feedback of the entire control loop. When the regulator is turned on automatically, the positive and negative effects of the regulator can be embodied.
The positive and negative effects of the valve positioner are determined according to the air opening and closing of the regulating valve, and the positive and negative effects of the regulator are determined according to the characteristics of each link of the control loop. It is necessary to ensure that the control loop meets the control requirements. For example, to realize negative feedback control. In the automatic control system, the adjusted parameter often deviates from the set value due to the influence of interference, that is, the adjusted parameter has a deviation:
For the regulator, according to the unified regulations, if the measured value increases, the regulator output increases, and the regulator amplification coefficient Kc is negative, the regulator is called a positive-acting regulator; the measured value increases, the regulator output decreases, When Kc is regular, the regulator is called a reaction regulator.
3. Selection of pneumatic control valve
Before any control system is put into operation, the positive and negative effects of the regulator must be correctly selected so that the direction of the control action is correct. Otherwise, the closed loop is not negative feedback but positive feedback. It will continue to increase the deviation, and eventually must Will lead the controlled variable to the highest or lowest limit value.
In a single-loop control system, as long as the product of the regulator's amplification factor Kc, the regulator valve's amplification factor Kv, and the controlled object's amplification factor Ko is positive, negative feedback control can be realized. The sign of the regulator, the control valve and the object amplification factor are specified as follows:
(1) The sign of the regulator's amplification factor; for regulators, in accordance with unified regulations, the measured value increases, the output increases, and the regulator's amplification factor Kc is negative, which is called positive action. The measured value increases, the output decreases, and Kc is positive, which is called a reaction.
(2) The sign of the amplification factor of the regulating valve; the amplification factor Kv of the regulating valve is defined as the air opening valve Kv is positive, and the air closing valve Kv is negative.
(3) The sign of the magnification coefficient of the object; the magnification coefficient Ko of the object is defined as: if the manipulated variable increases, the controlled variable also increases, Ko is positive; when the manipulated variable increases, the controlled variable decreases, and Ko is negative. It can be seen that the method for determining the positive and negative effects of the single-loop control system regulator is as follows: first determine the sign of the target amplification factor Ko, and then determine the positive or negative of the regulator's amplification factor Kv according to the selection of the regulating valve as air open or air closed Finally, the product of Kc, Kv, and Ko should be positive, which can determine the mode of action of the regulator.
In short, the choice of air-on and air-off is considered based on the safety point of process production. When the air supply is cut off, is the regulating valve safe in the closed position or in the open position? For example, for the combustion control of a heating furnace, a regulating valve is installed on the fuel gas pipeline, and the fuel supply is controlled according to the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this time, it is safer to choose an air-open valve, because once the air supply stops supplying, it is more appropriate for the valve to be closed than for the valve to be fully open. If the gas supply is interrupted and the fuel valve is fully opened, it will cause danger of overheating. Another example is a heat exchange equipment cooled by cooling water. The hot material exchanges heat with the cooling water in the heat exchanger to be cooled. The regulating valve is installed on the cooling water pipe. The temperature of the material after the heat exchange is used to control the amount of cooling water. When the air source is interrupted, the regulating valve should be in the open position, and it is safer to use the air-closed (ie FO) regulating valve.
4. Maintenance of pneumatic control valve
Pneumatic control valve is very important to ensure the normal operation and safe production of process equipment. Therefore, it is necessary to strengthen the maintenance of the pneumatic control valve.
A. Key inspection parts during maintenance
a. Check the inner wall of the valve: In the case of high pressure difference and corrosive media, the inner wall of the valve and the diaphragm of the diaphragm valve are often impacted and corroded by the medium, and the pressure and corrosion resistance must be checked;
b. Check the valve seat: due to the infiltration of the medium during work, the inner surface of the thread for fixing the valve seat is easily corroded and the valve seat is loose;
c. Check the spool: the spool is one of the movable parts of the regulating valve, and it is more severely eroded by the medium. When overhauling, carefully check whether the various parts of the spool are corroded and worn, especially under the condition of high pressure difference. The core wear is more serious due to cavitation caused by cavitation. Severely damaged spool should be replaced; check the packing.
B. Routine maintenance of pneumatic control valve
When the regulating valve uses graphite-asbestos as the packing, lubricating oil should be added to the packing once about three months to ensure that the regulating valve is flexible and easy to use. If it is found that the packing pressure cap is very low, the packing should be supplemented. If the PTFE dry packing is found to be hardened, it should be replaced in time; the operation of the regulating valve should be paid attention to during the patrol inspection, and the valve position indicator and regulator should be checked Whether the output is consistent; for the regulating valve with a positioner, check the air source frequently, and deal with the problem in time; always keep the regulating valve sanitary and the parts intact and easy to use.
Three common faults and their causes
(1) Failure and cause of non-operation of regulating valve
1. No signal, no gas source.
the reason:
â‘ The air source is not turned on;
â‘¡The air source is dirty, causing the air source pipe to be blocked or the filter and the pressure reducing valve to be blocked (pay special attention to the freezing of the air source with water in winter);
â‘¢Compressor failure causes low air source pressure;
â‘£ Leakage of gas source main pipe.
2. There is air source, no signal.
the reason:
â‘ The regulator is faulty, â‘¡the gas supply pipe is leaking; â‘¢the valve positioner is leaking; â‘£the diaphragm of the regulating valve is damaged.
3. The positioner has no air source.
the reason:
â‘ The filter is blocked; â‘¡The pressure reducing valve is faulty; â‘¢The pipeline is leaking or blocked.
4. The positioner has air source but no output.
the reason:
â‘ The orifice of the positioner is blocked; â‘¡The amplifier fails; â‘¢The nozzle is blocked.
5. There is signal, no action.
the reason:
â‘ The valve core falls off, â‘¡the valve core is stuck; â‘¢the valve stem is bent; â‘£the actuator spring is broken.
(2) Failures and causes of unstable operation of the regulating valve
1. The air source pressure is unstable.
the reason:
â‘ Leakage of gas source main pipe; â‘¡ Failure of pressure reducing valve.
2. The signal pressure is unstable.
the reason:
â‘ The time constant of the control system (T=RC) is not appropriate; â‘¡The output of the regulator is unstable.
3. The air source pressure is stable and the signal pressure is also stable, but the action of the regulating valve is still unstable.
the reason:
â‘ The ball valve of the amplifier in the positioner is not tightly closed due to dirt and wear, and the output will oscillate when the air consumption is particularly increased;
â‘¡The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle;
â‘¢The output pipe and line leak; â‘£The rigidity of the actuator is too small.
(3) Failures and causes of control valve vibration
1. The regulating valve vibrates at any opening.
the reason:
â‘ The support is unstable; â‘¡There is a vibration source nearby; â‘¢The valve core and bushing are seriously worn.
2. The regulating valve vibrates when approaching the fully closed position.
the reason:
â‘ The regulating valve is larger, and it is often used at a small opening; â‘¡The medium flow direction of the single seat valve is opposite to the closing direction.
(4) Failure and cause of slow action of the regulating valve
1. The valve stem is only sluggish when it moves in one direction.
the reason:
â‘ The diaphragm leaks in the pneumatic membrane actuator; â‘¡The "O" seal leaks in the actuator.
2. The valve stem is sluggish during reciprocating action.
the reason:
â‘ There is sticky clogging in the valve body; â‘¡There is a problem with the packing, which is too tight or needs to be replaced.
(5) Failures and reasons for the large leakage of the regulating valve that has been closed
1. Large leakage when the valve is fully closed.
the reason:
â‘ The valve core is worn out and the internal leakage is serious. â‘¡The valve is not properly adjusted and closed tightly.
2. The valve cannot reach the fully closed position.
the reason:
â‘ The medium pressure difference is too large, the rigidity of the actuator is small, and the valve is not closed tightly; â‘¡There is foreign matter in the valve; â‘¢The bush is sintered.
(6) The adjustable range of flow becomes smaller
The main reason is that the valve core is corroded and becomes smaller, so that the adjustable minimum flow becomes larger.
contact us
phone
WeChat: lfz13713759798
Official website:
mailbox:
Modular Jack section
China Keystone Jack,Ethernet Wall Jack,Ethernet Wall Plate,Rj45 Keystone, we offered that you can trust. Welcome to do business with us.
RJ45 interface can be used to connect RJ-45 connectors. It is suitable for the network constructed by twisted pair. This port is the most common port, which is generally provided by Ethernet hub. The number of hubs we usually talk about is the number of RJ-45 ports.
RJ45 is a type of different connectors (for example: RJ11 is also a type of connector, but it is used on the telephone); there are two different ways to arrange the RJ-45 connector: one is white orange, orange, white green, blue, white blue, green, white brown, brown; the other is white green, green, white orange, blue, white blue, orange, white brown, brown; therefore, there are also wires with RJ45 connector There are two kinds: straight line and interleaved line.
The RJ-45 port of the hub can be directly connected to terminal devices such as computers and network printers, and can also be connected with other hub equipment and routers such as switches and hubs. It should be noted that when connecting to different devices, the jumper method of twisted pair cable used is different.
Keystone Jack,Ethernet Wall Jack,Ethernet Wall Plate,Rj45 Keystone
ShenZhen Antenk Electronics Co,Ltd , https://www.antenkwire.com